Prerequisites for Implementing Cisco MPLS Forwarding

These prerequisites are required to implement MPLS Forwarding:

You must be in a user group associated with a task group that includes the proper task IDs. The command reference guides include
the task IDs required for each command. If you suspect user group assignment is preventing you from using a command, contact
your AAA administrator for assistance.

Router that runs
Cisco IOS XR software.

Installed composite mini-image and the MPLS package, or a full
composite image.

MPLS Forwarding Overview

MPLS combines the performance and capabilities of Layer 2 (data link layer) switching with the proven scalability of Layer 3
(network layer) routing. MPLS enables service providers to meet the challenges of growth in network utilization while providing
the opportunity to differentiate services without sacrificing the existing network infrastructure. The MPLS architecture is
flexible and can be employed in any combination of Layer 2 technologies. MPLS support is offered for all Layer 3 protocols,
and scaling is possible well beyond that typically offered in today’s networks.

Based on routing information that is stored in the VRF IP routing table and VRF CEF table, packets are forwarded to their
destination using MPLS.

A PE router binds a label to each customer prefix learned from a CE router and includes the label in the network reachability
information for the prefix that it advertises to other PE routers. When a PE router forwards a packet received from a CE router
across the provider network, it labels the packet with the label learned from the destination PE router. When the destination
PE router receives the labeled packet it pops the label and uses it to direct the packet to the correct CE router. Label forwarding
across the provider backbone, is based on either dynamic label switching or traffic engineered paths. A customer data packet
carries two levels of labels when traversing the backbone:

Top label directs the packet to the correct PE router

Second label indicates how that PE router should forward the packet to the CE router

Label Switching Functions

In conventional Layer 3 forwarding mechanisms, as a packet traverses the network, each router extracts all the information
relevant to forwarding the packet from the Layer 3 header. This information is then used as an index for a routing table lookup
to determine the next hop for the packet.

In the most common case, the only relevant field in the header is the destination address field, but in some cases, other
header fields might also be relevant. As a result, the header analysis must be done independently at each router through which
the packet passes. In addition, a complicated table lookup must also be done at each router.

In label switching, the analysis of the Layer 3 header is done only once. The Layer 3 header is then mapped into a fixed-length,
unstructured value called a label.

Many different headers can map to the same label, as long as those headers always result in the same choice of next hop. In
effect, a label represents a forwarding equivalence class—that is, a set of packets which, however different they may be,
are indistinguishable by the forwarding function.

The initial choice of a label need not be based exclusively on the contents of the Layer 3 packet header; for example, forwarding
decisions at subsequent hops can also be based on routing policy.

Once a label is assigned, a short label header is added at the front of the Layer 3 packet. This header is carried across
the network as part of the packet. At subsequent hops through each MPLS router in the network, labels are swapped and forwarding
decisions are made by means of MPLS forwarding table lookup for the label carried in the packet header. Hence, the packet
header does not need to be reevaluated during packet transit through the network. Because the label is of fixed length and
unstructured, the MPLS forwarding table lookup process is both straightforward and fast.

Distribution of Label Bindings

Each label switching router (LSR) in the network makes an independent,
local decision as to which label value to use to represent a forwarding
equivalence class. This association is known as a label binding.

Note

The distribution of label bindings cannot be done statically for the Layer 2 VPN pseudowire.

Each LSR
informs its neighbors of the label bindings it has made. This awareness of
label bindings by neighboring routers is facilitated by these protocols:

Label Distribution Protocol (LDP)

Supports MPLS forwarding along normally routed paths.

Resource Reservation Protocol (RSVP)

Supports MPLS traffic engineering.

Border Gateway Protocol (BGP)

Supports MPLS virtual private networks (VPNs).

When a labeled packet is sent from LSR A to the neighboring LSR B, the
label value carried by the IP packet is the label value that LSR B assigned to
represent the forwarding equivalence class of the packet. Thus, the label value
changes as the IP packet traverses the network.

MFI Control-Plane Services

The MFI control-plane provides services to MPLS applications, such as Label Distribution Protocol (LDP) and Traffic Engineering
(TE), that include enabling and disabling MPLS on an interface, local label allocation, MPLS rewrite setup (including backup
links), management of MPLS label tables, and the interaction with other forwarding paths (IP Version 4 [IPv4] for example)
to set up imposition and disposition.

MFI Data-Plane Services

The MFI data-plane provides a software implementation of MPLS forwarding
in all of these forms:

Imposition

Disposition

Label swapping

MPLS Maximum Transmission Unit

MPLS maximum transmission unit (MTU) indicates that the maximum size of the IP packet can still be sent on a data link, without
fragmenting the packet. In addition, data links in MPLS networks have a specific MTU, but for labeled packets. All IPv4 packets
have one or more labels. This does imply that the labeled packets are slightly bigger than the IP packets, because for every
label, four bytes are added to the packet. So, if n is the number of labels, n * 4 bytes are added to the size of the packet
when the packet is labeled. The MPLS MTU parameter pertains to labeled packets.

Label Security for
BGP Inter-AS Option-B

Option-B is a method
to exchange VPNv4/VPNv6 routes between Autonomous Systems (AS), as described in
RFC-4364. When a router configured with Option-B, peers with a router from
another confederation, or an autonomous system, and receives a labeled packet
from such an external peer, the router ensures the following:

the top label is
advertised to the source of traffic

label stack on the
packet received from the external peer contains at least one label (explicit
null label is not included)

Standards

Standards

Title

No new or modified standards are supported by
this feature, and support for existing standards has not been modified by this
feature.

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